<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .box{
        width: 200px;
        height: 200px;
        border: 1px solid red;
        position: relative;
    }
    .children-box{
        position: absolute;
        background-color: yellow;
        width: 100px;
        height: 100px;
        top:0;
        bottom:0;
        left: 0;
        right: 0;
        margin: auto;
    }
</style>
<body>
    <div id="app">
        <div class="box">
            <div class="children-box">

            </div>
        </div>
    </div>
</body>
</html>